Thuban is not a particularly bright star, but it holds a special place in the hearts of stargazers. That’s because Thuban – a relatively inconspicuous star in the constellation Draco the Dragon – was the Pole Star some 5,000 years ago, when the Egyptians were building the pyramids.
Among the many mysteries surrounding Egypt’s pyramids are the so-called “air shafts” in the Great Pyramid of Giza. These narrow passageways were once thought to serve for ventilation as the pyramids were being built. In the 1960s, though, the air shafts were recognized as being aligned with stars or areas of sky as the sky appeared for the pyramids’ builders 5,000 years ago.
One of the “air shafts” follows a crooked course through the Great Pyramid, so you couldn’t have sighted stars through it. To this day, the purpose of these passageways inside the Great Pyramid isn’t clear, although they might have been connected to rituals associated with the king’s ascension to the heavens. Whatever their purpose, the Great Pyramid of Giza reveals that its builders knew the starry skies intimately. They surely knew Thuban was their Pole Star, the point around which the heavens appeared to turn.
Past and future pole stars. Indeed, Thuban at times made a better pole star than our modern Polaris. Various sources claim that Thuban almost exactly pinpointed the position of the north celestial pole in the year 2787 B.C.
Meanwhile, our modern Polaris – which many centuries ago was an ordinary star known by the name Phoenice – won’t match Thuban’s precision when it most closely aligns with the north celestial pole on March 24, 2100. Polaris will be 27’09” (0.4525 degrees) from the north celestial pole at that time (a little less than the angular diameter of the moon when at its farthest from Earth), according to the computational wizard Jean Meeus.
The Northern Hemisphere also has had long stretches without a pole star. After the reign of Thuban but before that of Polaris, Kochab in the Little Dipper served as a rather poor pole star in 1100 B.C. Kochab was only half again as close to the north celestial pole as it is today.
Why does the identity of the pole star keep changing? Earth’s axis maintains a tilt that varies from about 22 degrees to 24 degrees from perpendicular every 41,000 years, with respect to the plane of our orbit around the sun. But, over a period of 26,000 years, Earth’s axis points out at different pole stars, tracing out a slow circle in the heavens. Whichever star lies on or near that circle will eventually be a pole star.
Many compare this motion of Earth – which is called precession or sometimes precession of the equinoxes – to that you sometimes see in a spinning top, which wobbles before it falls.
How to see Thuban. Thuban is part of the constellation Draco the Dragon. Although it’s not a super bright star, it is bright enough to see with relative ease on a dark night.
Most people star-hop to Thuban from the Big and Little Dippers. Draw an imaginary line that connects the stars Pherkad and Mizar. You’ll see Thuban, the former North Star, midway between these two guide stars.
Bottom line: Thuban was the Pole Star 5,000 years ago, when the Egyptian pyramids were being built. It is part of our constellation Draco the Dragon.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.